After almost eight years, my journey with the ACME experiment is finally coming to an end. On April 11, I will be defending my PhD dissertation titled “Progress on an Improved Measurement of the Electron Electric Dipole Moment”. This post provides an overview of what my thesis is about for a general audience, so that those of you who are interested in attending my defense or reading my thesis will hopefully not feel entirely lost.
In short, my thesis describes several components of the next generation ACME III experiment, which seeks to measure the shape of the electron’s charge (also known as the electron electric dipole moment or EDM) at least ten times more precisely than the ACME II experiment from five years ago.
You can download a copy of my thesis here. Or you can read this post first! A previous overview of the ACME experiment I wrote can also be found here.
Why is this important or interesting?
Good question! Measuring the electron EDM is a powerful way to search for new physics beyond the so-called “Standard Model” of particle physics. The Standard Model, or SM, has been the reigning paradigm of the field since the second half of the 20th century. Its predictions have been experimentally confirmed over and over in the laboratory – in fact, up to 12 decimal places in the case of the electron’s magnetic moment. You may have also heard of the Higgs boson, the last particle predicted by the SM which was finally discovered in 2012. However, the SM is also notorious for being bad at explaining other basic features of the universe such as there being more matter than antimatter. New theories of physics aim to rectify this by introducing new particles and/or interactions which can solve these conundrums. These theories often predict that the shape of the electron should be slightly distorted – meaning that the electron EDM should be non-zero. Thus, by measuring the electron EDM very precisely, one can verify or disprove the predictions of these new theories, shedding light on some of the most fundamental puzzles about the nature of physical reality.
So far, no non-zero EDM has been observed, which has both puzzled theorists and spurred experimentalists to keep looking for ways to improve the precision of their experiments. Perhaps the electron EDM is really non-zero – it’s just that it’s very, very small! Hence, after setting the world’s most stringent upper limit on the electron EDM in 2018 (about 10 times more precise than the previous limit, which was also set by ACME), the ACME collaboration has continued to look for ways to improve this result even further. Experiments like ACME are so precise that they are effectively probing physics with the same amount of precision as giant particle accelerators such as the Large Hadron Collider at CERN, Switzerland.
For more elaboration on the Standard Model, you can read an article I previously wrote about it for a general audience: The Frustrating Search for New Physics.
How does the ACME experiment measure the electron EDM?
In short, we measure the shape of the electron by subjecting electrons to a very strong electric field and observing their resulting behavior. But instead of attempting to do this with single, lone electrons, we use electrons embedded in certain special molecules which have the natural ability to amplify the electric field felt by the electron ten billion times stronger! The specific molecule we use is thorium monoxide (ThO). A beam of cold ThO gas is produced using a specially designed beam source. We then use an array of lasers, electric, and magnetic fields to manipulate and measure the properties of these molecules with extremely high precision. The basic measurement technique (also known as spin precession) is well-known and has remained the same since the ACME experiment first began about 15 years ago. However, for the next generation ACME III apparatus, many features have been vastly improved and optimized to keep decreasing the uncertainty of the measurement.
For more explanation on the basic measurement technique of the ACME experiment, you can read this post here.
Key advances in my thesis
The core of my thesis concerns several ACME III-related projects in which I was deeply involved in during my PhD:
- First is a measurement of the natural radiative lifetime of the H-state of ThO. To measure the electron EDM, the molecules must be excited into this state. How long the molecules remain there while in spin precession determines how well we can measure the EDM. In 2019-2021, I led an experiment measuring the lifetime, which found it to be about 4 ms, which is much longer than assumed in previous ACME experiments. Thus, for ACME III we made our experiment 5 times longer to let the molecules travel further while we are measuring the EDM. This reduces the uncertainty of the experiment by a factor of 2.6.
- Second are upgrades to the ACME photon detection system, which detects light from the molecules which we use to determine their properties when the spin precession measurement has concluded. In ACME II, only about 5% of all photons were detected. For ACME III, improvements in the optics we use to collect the photons (which I designed) and upgrades to the photon detectors (led by our Okayama University collaborators) has increased this to about 20%. This increase in signal also results in a marked improvement in our precision in measuring the EDM.
- Third are upgrades to the ACME data acquisition system. I developed the software and hardware of the experiment so as to enable it to acquire and store data at a ten times faster rate compared to before, or about 100 MB/s. This gives us much greater capabilities to understand the data taken in the experiment.
- Fourth is the design of ACME magnetic fields, comprising of the magnetic shields, coils, and magnetometry. My most extensive contributions are in the design of the new ACME coil system, which is used to apply the magnetic fields used in the spin precession experiment.
Summary and future outlook
Besides the projects above, other components of the ACME experiment have been constructed by members of the collaboration, such as a set of electrodes which will help to collimate the molecular beam, significantly increasing the number of molecules in the experiment. With all of these improvements put together, the precision of the ACME III experiment is projected to be at least an order of magnitude better than before. While the vast majority of individual components have been designed, tested, and manufactured, work is ongoing to assemble everything together. Unlike previous generations, the new experiment will be operated at Northwestern University, where I spent last summer wrapping up my experimental contributions to ACME:
I optimistically expect that in about 2-3 years time, ACME III will be able to complete yet another improved measurement of the electron EDM, which will probe physics at scales of many tens of TeV, demonstrating the immense prospects and capabilities of today’s smaller scale tabletop experiments to complement particle accelerators in exploring the cutting-edge frontier of our knowledge about the fundamental structure of the physical universe.
For more details, please feel free to attend my upcoming thesis defense!